1. Technical Field of the Invention
The present invention relates to a method and an apparatus for compression molding of a mixture of powder materials to manufacture ring-like pellets and to a dry cell, such as an alkaline-manganese dry cell, which contains pellets made of the powder mixture produced by the method and apparatus.
2. Description of Related Art
The market for alkaline-manganese dry cells has sharply been expanded with the spread of portable electronic appliances such as personal computers which consume a large amount of power. Alkaline-manganese dry cells which contain pellets made from a powder mixture are classified into six different types ranging from the standard R20 (D type) to a rectangular 9-Volt cell and are all fabricated in the form of a ring. These pellets are manufactured by compressing a mixture of powder materials in a ring-like mold with a compression molding machine, and hermetically loaded in a cell case.
The molding of such mixture pellets is generally performed with a rotary type compression molding machine as disclosed in Japanese Published Unexamined Patent Application No. 6-23597 or Japanese Published Utility Model Application No. 6-23694. The rotary compression molding machine of such type comprises a rotary disk 53 mounted by a bearing unit 52 to a center shaft 51 and driven by a drive unit 54 for rotation about the center shaft 51, as shown in FIG. 10. The rotary disk 53 carries at its circumferential edge a number of molding units 55 arranged at equal intervals. The molding unit 55 consists of a die 56, a lower plunger 57, and an upper plunger 58. After the powder material is supplied into the die 56 as the rotary disk 53 is rotated, it is compressed with the vertical movements of the lower plunger 57 and the upper plunger 58 which are actuated at predetermined locations by a lower pressing roller 59a and an upper pressing roller 59b, respectively. The formed pellet is pressed out and ejected from the die 56 by the upward movement of the lower plunger 57 which is actuated by a cam 60.
The conventional rotary compression molding machines disclosed in the above mentioned publications are designed for forming disk-like pellets. For molding a ring-like pellet, the die has to be replaced with an appropriate one equipped with a center pin.
A forming operation in a conventional rotary compression molding machine being constructed as mentioned above is now explained referring to FIGS. 11 and 12. As shown in a longitudinal sectional view of the rotary compression molding machine of FIG. 11, a rotary disk 31 has a plurality of molding units 32 arranged at equal intervals on a concentric circle about the center of rotation. Each molding unit 32 comprises a die 33, a center pin 34, a lower plunger 35, and an upper plunger 36. The die 33 is fixed to the rotary disk 31 and the center pin 34 is fitted into an axial bore of the lower plunger 35 for sliding movement in relation to the lower plunger 35. The lower plunger 35 and the upper plunger 36 are arranged to engage with a lower pressing roller 38 and an upper pressing roller 39 respectively at their corresponding locations as the rotary disk 31 is rotated so as to compress the powder material filled in an annular space between the die 33 and the center pin 34 from upper and lower sides to form a ring-like pellet 40. The molded ring-like pellet 40 is then pressed upwardly out from the die 33 by the lower plunger 35 which is greatly lifted up by the engagement with a cam 41.
A procedure of forming the pellet 40 with the rotary compression molding machine described above is explained in more detail referring to FIG. 12. FIG. 12A illustrates an initial state where the die 33, the lower plunger 35, and the center pin 34 are flush with each other at the top after the previous pellet 40 is unloaded. When the lower plunger 35 is lowered from its initial position, an annular space for compression molding is formed between the die 33 and the center pin 34 as shown in FIG. 12B. The annular space is then filled with a powder material 42. As a feed shoe 43 runs along the top sides of the die 33 and the center pin 34 located flush with each other, an excess of the powder material 42 is removed to measure out a predetermined amount to be molded into one pellet 40. This is followed by a step where the lower plunger 35 is lifted up and the upper plunger 36 is lowered as shown in FIG. 12D, by which the powder material 42 in the annular space is compressed from upper and lower sides, thus forming the pellet 40. The pellet 40 is then unloaded upwardly from the die 33 by the upward movement of the lower plunger 35 as shown in FIG. 12E and taken out as a compression molded product.
Such conventional procedure of compression molding has, however, a drawback that the powder material 42 when being supplied into the annular space between the die 33 and the center pin 34 is likely to produce a bridge, particularly when a thin pellet 40 having a small diameter is formed. Because of the bridges frequently formed, it is difficult to constantly supply a given amount of the powder material 42, thus making the weight of pellet 40 unstable. In order to feed a fixed amount of the powder material 42 into the die 33, it is of course attempted to destroy the bridge by stirring the heap of the powder material 42 on the die 33 with a plurality of feed shoes 43 provided at an angle to the direction of movement of the die 33 and driven by the rotation of the rotary disk 31. This attempt at eliminating the bridge is yet insufficient to fully prevent the variation in weight and height of the pellets 40.
The pellet 40 formed in the compression molding is pressed upward and unloaded from the space defined by the stationary center pin 34 and the die 33 by the ejecting action of the lower plunger 35. Since the pellet 40 is stuck to the center pin 34 and the die 33 at the inner side and outer side thereof respectively by the pressure given during the compression molding, the lower plunger 35 is required to have a considerable amount of strength to unload the pellet 40 by pushing it up. This causes severe abrasion on the sliding surfaces between the lower plunger 35 at its bottom and the surface of the cam 41 which functions to lift up the lower plunger 35 as the rotary disk 31 rotates.
Further, the pellet 40 is forcibly unloaded by the ejecting action of the lower plunger 35 though it is almost fixedly stuck to the inner side of the die 33 and the outer side of the center pin 34. For preventing the pellet 40 from being damaged during the removal from the die 33, the pellet 40 is required to be tapered both on its inner and outer sides at a relatively wide angle. When the pellet 40 of a ring-like shape is tapered both on the inner and outer sides, its overall weight is decreased. This cannot be compensated by setting the height of the pellet 40 vertical to the radial direction to be greater, because the lower part of the pellet 40 becomes too small in thickness due to the tapering.
Since the pellet 40 is small both in height h and weight, three or four pellets 40 are needed for filling a cell case 62 as a positive electrode material to construct an alkaline-manganese dry cell 61 of R20 to R03 types as shown in FIG. 13. As the number of the pellets 40 to be encased increases, more steps are needed for compression molding and filling process, thus declining the efficiency of production and soaring the overall cost. The gaps made between the inner side of the pellet 40 and a separator 64 and between the outer side of the pellet 40 and the cell case 62 obstruct the smooth flow of the electric current. Also shown in FIG. 13 are a label cover 63, a gel negative electrode 65, a collector 66, a resin seal 67, insulators 68, and a bottom cap 69.
In general, the ring-like pellets are made from a mixture material by a compression molding machine as described above and transferred by belt conveyors or parts feeders to the next step of loading with an automatic loader. Those steps are hardly carried out at a higher speed thus being low in productivity. Also, a system including the compression molding machine, the conveyors, and the automatic loader is bulky requiring a large installation area and increasing the facility cost. While the pellets are being transferred by the conveyors, they may suffer from vibrations and shakes resulting in physical damages. If chips peel off from the pellets, they may be scattered around the system thus impairing the environment of a working site.
In view of the foregoing, it is an object of the present invention to provide a method and an apparatus for molding a powder material by compression which is capable of stably forming pellets being constant in weight and height, and of ejecting out a formed pellet from a die with a little amount of force, and has high productivity with less abrasion of components and less possibility of damaging the pellets.
It is also an object of the present invention to provide a dry cell which contains in its case a less number of pellets, of which inner and outer sides are less or not tapered, thus ensuring high capacity and high performance of supplying a higher current and contributing to the improvement of productivity.
It is another object of the present invention to provide a rotary type powder compression molding assembly system being capable of both forming a pellet by compression molding and inserting it into a case thus increasing the productivity, minimizing the space requirement and the cost, and improving the quality of pellets and the performance of a dry cell.
To accomplish the above said object, in a compression molding method for compressing a powder material filled in an annular molding space defined between a cylindrical die and a center pin mounted in the axial center of the die with a lower plunger and an upper plunger to form a ring-like pellet, the present invention is characterized in that the powder mixture is supplied into the die under a state that the center pin is located lower than the top surface of the die, after which the center pin is lifted up to a given molding position to define the annular molding space between the center pin and the die, and the powder mixture in the annular molding space is compressed from upper and lower sides with the upper plunger and the lower plunger.
The powder mixture can be thereby fed into the die without making any bridges. Since the center pin is lifted up to its molding position after the die is filled with the powder mixture, the annular molding space can be filled with a constant amount of the powder mixture, whereby pellets of uniform weight and height can be stably formed.
After the compression molding of the powder mixture, the center pin is unitedly lifted up with the lower plunger so as to push up the formed pellet out of the die, after which the pellet is taken out from the center pin. Accordingly, the pellet with the center pin can easily be pushed out of the die with a small amount of force. The pellet released from a compressing stress of the die is stuck to the center pin with a less force, thus can be readily taken out from the center pin without a wide tapering angle at the side thereof. Further, a given amount of the powder mixture is measured out while the center pin is immersed in the powder mixture, after which the lower plunger is lowered to cause the top surface of the powder mixture to sink lower than the top surface of the die, while the upper plunger is lowered to close an opening of the die with the lower end thereof, and the center pin immersed in the powder mixture is vertically reciprocated while being lifted up to its molding position, where the top end thereof emerges from the top surface of the powder mixture, and where the annular molding space is defined between the center pin and the die.
Since the opening of the die is closed by the upper plunger and the center pin is vertically reciprocated during the upward movement, the powder mixture remaining on the center pin is completely removed. As the annular molding space is filled with the precise amount of the powder mixture, pellets of uniform weight and height can be stably formed. In particular, the center pin may have a sharp tip for ease of the removal of the remaining powder mixture. The powder can be precisely measured out without being disturbed by such configuration of the center pin, as it is immersed in the powder mixture when the measurement is made.
To implement the above described compression molding method, a powder compression molding apparatus of the present invention has a molding unit comprised of a cylindrical die, a center pin disposed at the axial center of the die, and a lower plunger and an upper plunger for compressing a powder mixture filled in an annular molding space defined between the die and the center pin, and is characterized in that the lower plunger and the center pin are arranged to be movable in the axial direction in relation to each other as well as to the die, and that actions of the lower plunger and the center pin are separately controlled by an operation controller.
More specifically, the apparatus further comprises a first lower plunger actuating cam means for driving the lower plunger to perform a compression molding action in the die, a second lower plunger actuating cam means for driving the lower plunger to perform a vertical motion in the die, a third lower plunger actuating cam means for carrying out an ejecting operation to push the pellet upwardly out of the die, and a center pin actuating cam means for driving the center pin to ascend to a molding position after being moved downwardly lower than the top surface of the die, and to perform the ejecting operation to push the pellet out from the die, by which a series of molding actions can be carried out as the molding unit moves.
Further, the apparatus may further comprise a lower lifting shaft, to the upper end of which the lower plunger is mounted and within which the center pin is coaxially disposed for relative sliding movements, the lower lifting shaft being provided with a first cam follower at a lower end thereof to engage with a lower pressure roller and with a second cam follower at a middle part thereof to engage with a lower plunger actuating cam, and a third cam follower connected to the center pin to engage with a center pin actuating cam, whereby the lower pressure roller and the first cam follower constitute a first lower plunger actuating cam means, the lower plunger actuating cam and the second cam follower constitute a second lower plunger actuating cam means, and the center pin actuating cams and the third cam follower constitute a center pin actuating cam means.
By the engagement between the first cam follower at the lower end of the lower lifting shaft and the lower pressure roller, a great amount of load needed for compression molding action can be smoothly transmitted to the lower plunger. Also, the engagement between the cam and the second and third cam followers mounted to a side of the middle part of the lower lifting shaft allows the lower plunger and the center pin to be operated at a high speed.
The center pin actuating cam means may comprise a center pin actuating cam having a cam surface only at a lower side thereof, the cam follower joined to the center pin and engaged with the cam surface of the center pin actuating cam, and a resilient member for constantly urging the center pin upwardly and detachably pressing the cam follower against the cam surface of the center pin actuating cam. The center pin can thereby be downwardly retracted upon receiving the great load of compression molding action by contraction of the resilient member, positively preventing damages or bend of a cam follower shaft of the center pin. The resilient member also ensures the engagement between the cam follower and the center pin actuating cam, assisting the center pin to smoothly perform the necessary actions.
Also, the center pin actuating cam means may be so constructed that the center pin is vertically reciprocated more than once during the step of lifting up the center pin from its lowered position to the molding position. Any residual powder mixture on the center pin can thereby be effectively cleared away as the center pin is lifted up from the powder mixture, so that a precise amount of the powder mixture can remain in the annular molding space.
The center pin may comprise an upper portion for defining the annular molding space with the die and a lower portion which is smaller in diameter than the upper portion, and a powder outlet space may be provided between the lower portion and the lower plunger, so that the powder mixture which entered into the clearance between the lower plunger and the center pin moving in relation to each other can be automatically and quickly discharged, preventing abrasion or increase in frictional resistance between the lower plunger and the center pin. Thus, smooth relative movements of the lower plunger and the center pin can be guaranteed without frequent maintenance operations.
At least the outer side of the center pin or both of the inner side of the die and the outer side of the center pin may be perpendicularly constructed to give the pellet a more cylindrical configuration. Pellets which are great in height and have little differences in thickness between their upper and lower ends can thereby be molded. Also, such pellets can be readily unloaded according to the above described method.
The center pin may have a sharp end of a bullet-nose shape or a polygonal conical shape provided at the top end thereof. This allows the residual powder mixture on the center pin to be effectively removed while the center pin is lifted up to emerge from the powder mixture, thus keeping a precise amount of the powder mixture in the annular molding space.
Further, a plurality of the molding units are mounted at equal intervals on a circle about the center of rotation of a rotary disk, so that pellets can be continuously and time-effectively produced with the rotating movement of the rotary disk with a simple and compact structure.
A dry cell of the present invention contains a powder mixture pellet which is formed to be of a ring-like shape by the above described compression molding method and has an inner side or both inner and outer sides thereof not being tapered thus having a cylindrical configuration. As the differences in thickness of the powder mixture pellet at its upper and lower ends are minimized, a necessary quantity of powder mixture for meeting requirements of the dry cell can be sufficed with a fewer number of pellets. Such dry cell is capable of supplying a greater amount of electric current, as there is no large gaps between the inner side of the pellets and a separator or between the outer sides of the pellets and the case.
A dry cell of any types from R20 to R1 may be constructed to have one or two powder mixture pellets contained in a cell case which are formed by the above described compression molding method, thereby decreasing the number of steps for inserting the pellets into the case to increase productivity and to reduce the overall cost.
In a rotary type powder compression molding assembly system according to the present invention, a plurality of molding units for producing pellets from a powder mixture by compression molding are mounted at equal intervals on a circle about the center of rotation of a rotary disk, and an insertion assembly station is mounted at an appropriate position on a movement path of the molding units for inserting the molded pellet into a case, thereby carrying out compression molding action and insertion assembling action with a single system at a high speed, thus increasing productivity and decreasing space requirement and the facility cost. As there are fewer steps for transferring the pellets, the pellets are less likely to be damaged during the transfer, increasing the quality of product.
A plurality of the insertion assembly stations may be provided so that the pellets formed at each of the molding units located between the insertion assembly stations are inserted into the case immediately after the compression molding at the next insertion assembly stations, thereby it is possible to load the pellets into the cases at two or more different locations, or to load a plurality of pellets into one case in a single assembly system, helping to further increase the speed of operation.
The insertion assembly station may be provided in a pair, and may further comprise a case carrying-in means for feeding the cases into one insertion assembly station, a series of case holding means for holding and conveying the cases loaded with the pellet to another insertion assembly station, and a case carrying-out means for removing the cases after being loaded with the pellet at another insertion assembly station. This allows the case to be filled with two or more pellets in a single assembly system, thus increasing the speed of production.
Each of the case holding means may be mounted on the rotary disk corresponding to each molding unit and be constructed to hold and cause the case loaded with the pellet at the first insertion assembly station to return to its retracted position beside the molding unit, and to advance the case to the movement path of the molding units at the next insertion assembly station. Accordingly, the cases can be readily transferred into the succeeding insertion assembly station.
The molding unit may comprise a substantially cylindrical die, a center pin mounted in the axial center of the die, and a lower plunger and an upper plunger for molding a powder mixture filled in an annular molding space defined between the die and the center pin, so that the pellet is assembled into the case by being pushed up into the case located coaxially above the die by the action of both the lower plunger and the center pin, and lowering the center pin thereafter while the pellet is supported by the lower plunger. This permits the molding unit to be used as an insertion assembling means thus contributing to the simple construction of the system.
The case holding means may be mounted to one end of an operating lever which is mounted on the rotary disk corresponding to each molding unit, the operating lever being rotatably connected to the rotary disk with a cam follower at the other end thereof engaged with a cam disposed coaxially with the rotary disk, the cam having a retraction cam surface for holding the case holding means at its retracted position beside the molding unit and an operating cam surface for causing the case holding means to advance to and retract from the movement path of the molding unit. Accordingly, the case holding means can be controlled by a simple construction of cam arrangements to move between the position on the movement path of the molding units and the retracted position as the rotary disk rotates, allowing the formed pellets to be loaded into the case smoothly and continuously.
These and other objects, features and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention in conjunction with the accompanying drawings.